1. Field of the Invention
This invention relates to a stator for an electric motor; and more particularly, to an amorphous metal stator for a high efficiency radial-flux electric motor.
2. Description of the Prior Art
A radial-flux design electric motor typically contains a generally cylindrical stator made from a plurality of stacked laminations of non-oriented, electrical steel. Each lamination has the shape of a circular washer having “teeth” that form the poles of the stator. The teeth protrude from the inner diameter of the stacked laminations and point toward the center of the cylindrical stator. Each lamination is typically formed by stamping, punching or cutting the mechanically soft, non-oriented electrical steel into the desired shape. The formed laminations are then stacked and bound to form a stator.
Although amorphous metals offer superior magnetic performance when compared to non-oriented electrical steels, they have long been considered unsuitable for use in electric motors due to certain physical properties and the corresponding fabricating limitations. For example, amorphous metals are thinner and harder than their non-oriented steel counterparts and consequently cause fabrication tools and dies to wear more rapidly. The resulting increase in the tooling and manufacturing costs makes fabricating amorphous metal stators using such techniques commercially impractical. The thinness of amorphous metals also translates into an increased number of laminations in the assembled stator, further increasing the total cost of an amorphous metal stator.
Amorphous metal is typically supplied in a thin continuous ribbon having a uniform ribbon width. However, amorphous metal is a very hard material making it very difficult to cut or form easily, and once annealed to achieve peak magnetic properties, becomes very brittle. This makes it difficult and expensive to use conventional approaches to construct an amorphous metal magnetic stator. The brittleness of amorphous metal also causes concern for the durability of a motor or generator which utilizes amorphous metal magnetic stators. Magnetic stators are subject to extremely high magnetic forces which change at every high frequencies. These magnetic forces are capable of placing considerable stresses on the stator material which may damage an amorphous metal magnetic stator.
Another problem with amorphous metal magnetic stators is that the magnetic permeability of amorphous metal material is reduced when it is subjected to physical stresses. This reduced permeability may be considerable depending upon the intensity of the stresses on the amorphous metal material. As an amorphous metal magnetic stator is subjected to stresses, the efficiency at which the core directs or focuses magnetic flux is reduced resulting in higher magnetic losses, reduced efficiency, increased heat production, and reduced power. This phenomenon is referred to as magnetostriction and may be caused by stresses resulting from magnetic forces during the operation of the motor or generator, mechanical stresses resulting from mechanical clamping or otherwise fixing the magnetic stator in place, or internal stresses caused by the thermal expansion and/or expansion due to magnetic saturation of the amorphous metal material.
Non-conventional approaches to amorphous metal stator designs have been proposed. In one approach, a “toothless” stator, consisting simply of a tape-wound amorphous metal toroid, has been suggested. While this approach produces an efficient motor, the large air gap between the stator and rotor limits the performance and control of the motor. A second approach attempts to replicate the conventional stator shape by combining a tape-wound amorphous metal toroid with stacks of cut amorphous metal. The wound amorphous metal toroid forms the back-iron of the stator and the cut amorphous metal stacks are mounted on the inner diameter of the toroid to form the teeth or poles. While this approach reduces the air gap between the stator and rotor, the magnetic flux must cross the many layers of tape wound back-iron as the flux passes from the tooth to the back-iron. This greatly increases the reluctance of the magnetic circuit and the electric current required to operate the motor.
A third approach, disclosed by U.S. Pat. No. 4,197,146 to Frischmann, fabricates the stator from molded and compacted amorphous metal flake. Although this method permits fabrication of complex stator shapes, the structure contains numerous air gaps between the discreet flake particles of amorphous metal. This will greatly increase the reluctance of the magnetic circuit and the electric current required to operate the motor.
A fourth approach, taught by German Patents DE 28 05 435 and DE 28 05 438, divides the stator into wound pieces and pole pieces. A non-magnetic material is inserted into the joints between the wound pieces and pole pieces, increasing the gap, and thus the reluctance of the magnetic circuit and the electric current required to operate the motor. The layers of material that comprise the pole pieces are oriented with their planes perpendicular to the planes of the layers in the wound hack iron pieces. This configuration further increases the reluctance of the stator, because contiguous layers of the wound pieces and of the pole pieces meet only at points, not along full line segments, at the joints between their respective faces. In addition, this approach teaches that the laminations in the wound pieces are attached to one another by welding. The use of heat intensive processes, such as welding, to attach amorphous metal laminations will recrystallize the amorphous metal at and around the joint. Even small sections of recrystallized amorphous metal will increase the magnetic losses in the stator.
A fifth approach, disclosed by U.S. Pat. No. 2,255,684 to Mischler, involves fabricating the stator from either laminated strips of amorphous metal or from moldable composites of amorphous metal flake. Stator fabrication using laminated strips involves bending the strips to the desired stator shape. The mechanical stresses imparted on the laminated strips during the shaping operation will increase the core loss of the finished stator. This approach does not envision annealing of the stator to relieve the mechanical stresses. Stators fabricated from moldable composites of amorphous metal flake will contain numerous air gaps between the discreet flake particles. This will greatly increase the reluctance of the magnetic circuit and the electric current required to operate the motor.